From e7137ec157b454bead2993a38a0069eba7d3cab7 Mon Sep 17 00:00:00 2001
From: John Litborn <11260241+jakkdl@users.noreply.github.com>
Date: Wed, 18 Jan 2023 19:23:55 +0100
Subject: [PATCH] B906: ignore functions whose _fields attribute can't contain
 ast.AST subnodes (#335)

---
 README.rst    |  5 +++++
 bugbear.py    | 25 +++++++++++++++++++++----
 tests/b906.py | 30 ++++++++++++++++++++++++++++++
 3 files changed, 56 insertions(+), 4 deletions(-)

diff --git a/README.rst b/README.rst
index 8de3e44..22f7cb7 100644
--- a/README.rst
+++ b/README.rst
@@ -308,6 +308,11 @@ MIT
 Change Log
 ----------
 
+Future
+~~~~~~~~~
+
+* B906: Ignore ``visit_`` functions with a ``_fields`` attribute that can't contain ast.AST subnodes. (#330)
+
 23.1.17
 ~~~~~~~~~
 
diff --git a/bugbear.py b/bugbear.py
index ba1eefc..aad3557 100644
--- a/bugbear.py
+++ b/bugbear.py
@@ -997,12 +997,29 @@ def check_for_b906(self, node: ast.FunctionDef):
         if not node.name.startswith("visit_"):
             return
 
-        # extract what's visited, only error if it's a valid ast subclass
-        # with a non-empty _fields attribute - which is what's iterated over in
-        # ast.NodeVisitor.generic_visit
+        # extract what's visited
         class_name = node.name[len("visit_") :]
         class_type = getattr(ast, class_name, None)
-        if class_type is None or not getattr(class_type, "_fields", None):
+
+        if (
+            # not a valid ast subclass
+            class_type is None
+            # doesn't have a non-empty '_fields' attribute - which is what's
+            # iterated over in ast.NodeVisitor.generic_visit
+            or not getattr(class_type, "_fields", None)
+            # or can't contain any ast subnodes that could be visited
+            # See https://docs.python.org/3/library/ast.html#abstract-grammar
+            or class_type.__name__
+            in (
+                "alias",
+                "Constant",
+                "Global",
+                "MatchSingleton",
+                "MatchStar",
+                "Nonlocal",
+                "TypeIgnore",
+            )
+        ):
             return
 
         for n in itertools.chain.from_iterable(ast.walk(nn) for nn in node.body):
diff --git a/tests/b906.py b/tests/b906.py
index 35cfe42..f947c14 100644
--- a/tests/b906.py
+++ b/tests/b906.py
@@ -54,3 +54,33 @@ def a():
 
 def visit_():
     ...
+
+
+# Check exceptions for ast types that only contain ADSL builtin types
+# i.e. don't contain any ast.AST subnodes and therefore don't need a generic_visit
+def visit_alias():
+    ...
+
+
+def visit_Constant():
+    ...
+
+
+def visit_Global():
+    ...
+
+
+def visit_MatchSingleton():
+    ...
+
+
+def visit_MatchStar():
+    ...
+
+
+def visit_Nonlocal():
+    ...
+
+
+def visit_TypeIgnore():
+    ...